Tuesday, July 7, 2009

Coldest Known Objects in Space concept of Planck

Tuesday, July 7, 2009
The detectors of Planck's High Frequency Instrument reached their amazingly low operational temperature of -273.05 degrees Celsius (-459.49 degrees Fahrenheit), making them the coldest known objects in space. The spacecraft has also just entered its final orbit around the second Lagrange point of the sun-Earth system, called L2.

For more information, go to the story on ESA’s Planck site at http://www.esa.int/SPECIALS/Planck/SEM0Y5S7NWF_0.html

Coolest spacecraft ever in orbit around L2

Located in the focal plane of the telescope, Planck’s Low Frequency Instrument (LFI), and the High Frequency Instrument (HFI), are equipped with a total of 74 detectors covering nine frequency channels. These detectors must be cooled to temperatures around or below 20 K so that their heat does not swamp the faint microwave signals they are designed to detect.

The first part of this animation shows the different stages of Planck’s active cooling system. This active cooling system consists of a three-stage refrigeration chain which takes over after the passive cooling system cools the telescope to about 50 K.

The first stage makes use of liquid hydrogen to reduce the temperatures to 20 K. The second stage is a mechanical cooler (a pump) that uses liquid helium (4He) to bring the temperatures down to 4 K. The third stage makes use of a mixture of Helium 3 and Helium 4 (3He and 4He) to reach an amazing low temperature of just 0.1 K.

The second part of the animation shows microwave light collected by telescope reaching the instrument detectors via the conical feed horns. The bolometric detectors of the HFI, located behind the horns, absorb the light and heat up slightly.

Like measuring the heat of a rabbit on the Moon

This artist's impression shows the focal plane unit of the Planck telescope.

Planck's instruments will operate at a few degrees above absolute zero. To achieve this, a series of cooling stages are required. Without any additional cooling, the spacecraft itself can passively cool to around 50 K (about -223ºC).

For the Low Frequency Instrument (LFI) the focal plane is cooled to around 20 K (about -253ºC). The signals received in the instrument horns are amplified and passed through waveguides to a back-end unit (which is at around 300 K, or about 27ºC) where the signals are processed.

The High Frequency Instrument (HFI) unit is more compact and sits entirely within the LFI housing at about 18 K (about -255ºC). The bolometers (devices for measuring incident electromagnetic radiation) are then stage cooled to just 0.1 K (-273.14ºC) with the detected signals again processed in a warmer back-end readout and data processing unit.

Isolating the various components and cooling only small volumes to the coldest temperatures the optimum efficiencies and mission achieve lifetime.

Planck's orbit around L2, the second Lagrange point of the Sun-Earth system.

Credits: ESA

This artist's view shows the combined focal plane of the two instruments on board ESA's Planck spacecraft. The High Frequency Instrument (HFI) is visible as a circular forest of horns at the centre, surrounded by the Low Frequency Instrument (LFI) ring of horns. The two instruments detect the collected radiation in different ways.

LFI is designed to convert the lower energy microwaves into electrical voltages, rather like a transistor radio. HFI works by converting the higher energy microwaves to heat, which is then measured by a tiny electrical thermometer. The instruments share a common telescope.

Science operations to begin soon

Planck will scan the entire sky to build the most accurate map ever of the Cosmic Microwave Background (CMB), the relic radiation from the Big Bang. The spacecraft will spin at 1 rotation per minute around an axis offset by about 85° so that the observed sky region will trace a large circle on the sky. As the spin axis follows the Sun the circle observed by the instruments sweeps through the sky at a rate of 1° per day. Planck will take about 6 months to complete a full scan of the sky, allowing the creation of two complete sky maps during the nominal mission lifetime (about 15 months).


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